1. Field of the Invention
The invention relates to a powder-spraying appliance for electrostatic powder coating having a chamber supplied with an air/powder mixture via a supply conduit. At least one high-voltage electrode and an earth electrode are disposed within the chamber for charging the powder.
German Patent DE 27 22 100 C2 discloses a powder-spraying appliance containing at least one chamber. In the chamber, there are disposed a point-discharge electrode opposite to a flow of material and an obtuse-angled counter-electrode having an opposite polarity to the point-discharge electrode and is located upstream at a distance from the latter. In order to improve the charging of the powder particles, a configuration with a plurality of chambers connected in series is provided. It is proposed, moreover, to configure the chamber so as to result continuously in widenings and narrowings which are intended to ensure that material particles are retarded, at the same time being swirled, and are therefore exposed more intensively to the force field. Finally, it is proposed to arrange, so as to be distributed over the circumference of the at least one chamber, orifices, via which additional air is introduced and which reduce the average density of the mixture of air and powder and improve the charging.
Tests have shown that the measures are not sufficient to bring about a coating of powder on a work piece with adequate efficiency. Since the electrodes come directly into contact with the powder, powder is deposited on the electrodes, thus resulting in markedly poorer charging.
It is pointed out, in U.S. Pat. No. 4,811,898, that, in a spraying appliance with a baffle body, the relative position of the baffle body and the electrodes is an essential parameter. Use of external electrodes may be gathered from U.S. Pat. No. 4,228,961.
A powder-spraying appliance is known from Published, European Patent Application EP 0 237 249. The appliance has a chamber, the cross-section of which is larger than the cross-section of a supply conduit for the powder/air mixture. An earth electrode and a plurality of high-voltage electrodes, onto which cleaning air can be blown are disposed in the chamber.
The configuration of the chamber and, in particular, of the electrodes is selected in such a way that a plasma with positive polarity and a plasma with negative polarity are generated. A voltage is applied intermittently to the plasma electrodes. Tests have shown that these measures result in sparking and in the fouling of the electrodes.